Sucrose is of central importance for the plant and serves many functions. For the long distance transport of photoassimilates and/or energy between various organs in plants, sucrose is almost exclusively used. The sucrose, which is transported in a specific heterotrophic organ determines the growth and the development of this organ. Thus it is known, e.g. from EP 442 592, that transgenic plants, in which the transport of the sucrose away from the exporting leaves is inhibited by expression of an apoplastic invertase, shows a strong reduction in the growth of e.g. roots or tubers in the case of potato plants. For tobacco plants, the principal importance of sucrose for the long distance transport of energy carriers within the plant is described in von Schaewen et al, 1990, EMBO J 9: 3033-3044.
While it has been clearly shown that a reduction of the amount of sucrose imported in the heterotrophic organs, such as tubers and seeds, leads to loss of yield, it is not known whether an increase in the amount of sucrose in the photosynthetically active parts of the plant, mainly the leaves, leads to a better supply of heterotrophic organs and thus to an increase in yield.
A second central role for sucrose and/or the hexoses, glucose and fructose which are derived from sucrose, is in the protection of plants against frost damage at low temperatures. Frost damage is one of the main limiting factors in agricultural productivity in the northern hemisphere. Temperatures below freezing lead to the formation of ice crystals. Since the growing ice crystals consist of pure water, water is abstracted from the cells as the temperature falls.
This dehydration has at least two potential damaging results:
a) all dissolved substances within a cell are strongly concentrated and the cell contracts following the loss of water. Highly concentrated salts and organic acids lead to membrane damage; PA1 b) with rehydration from dew, the previously contracted cells reexpand. The cell membrane also expands again. The volume expansion puts a heavy mechanical load on the membrane. PA1 1 as the transport form for the distant transport of photoassimilates, PA1 2 as an osmotically active substance with the desirable activity of lowering the freezing point in intact, growing plants, and PA1 3 in the undesirable formation of reducing sugars in stored harvested parts of a plant, e.g. the potato tubers, as a result of low temperatures. PA1 a) a suitable promoter that ensures that the coding sequence is read off at the suitable time point and/or in a specified development stage in the transgenic plants or in specified tissues of transgenic plants, PA1 b) at least one coding sequence, that PA1 c) a non-coding termination sequence that contains the signals for the termination and polyadenylation of the transcript.
It is thus clear that a freezing/dew cycle can lead to severe membrane damage of the cells and thus to damage to the plant.
It thus appears worth while to hinder the freezing of plant cells. One possible strategy is to increase the formation of osmotically active substances in the cytosol of plant cells. This should lead to a lowering of the freezing point. Osmotically active substances include sucrose and/or the two hexoses which are derived from sucrose.
The increased formation of sucrose and/or the two hexoses at low temperatures is desirable in the growing plant. Another situation can exist in the harvested parts of a plant, especially in storage. For example, in potato tubers that are stored at 4-8.degree. C., hexoses (glucose) accumulate. It would appear to be sensible, to see this as the answer to a lowering of the temperature ("cold-sweetening").
The accumulation of sucrose and glucose has in the case of potato tubers economically undesirable results. Increased amounts of reducing sugars, such as glucose, in potatoes which are fried when preparing crisps, chips and the like, leads to an undesirable browning due to the Maillard reaction. Such products with a dark brown colour are not generally acceptable to the consumer. Further the cooking strength is strongly dependent on the content of starch and/or its breakdown products which are important in determining the quality characteristics of the potato.
In relation to the economic aspects, sucrose thus possesses three especially important functions:
The biosynthesis pathways for the formation of sucrose, either from the primary photosynthesis products (in the leaf) or by breakdown of starch (in the storage organs e.g. of potatoes), are known. An enzyme in sucrose metabolism is sucrose-phosphate-synthase (SPS). It forms sucrose-6-phosphate from UDP-glucose and fructose-6-phosphate, which in a second step is converted to sucrose.
The isolation of SPS from maize and the cloning of a cDNA from mRNA from maize tissue is known (EP 466 995). In this application, processes for the purification of a protein such as by centrifuging of homogenates, differential precipitation and chromatography are described. A 300 times enrichment of SPS from plant tissue has been described by Salerno and Pontis (Planta 142: 41-48, 1978).
In view of the significance of SPS for carbohydrate metabolism it is questionable whether plants can tolerate a reduction in SPS activity in all or in certain organs. It is especially not known whether it is possible to produce transgenic plants with a reduced SPS activity. Also the use of SPS for the modification of the functions of sucrose for lowering the freezing point in intact plants and for the formation of reducing sugars in harvested parts is not known.
For the preparation of plants with reduced SPS activity, i.e. plants with changed sucrose concentration, it is necessary to make available an SPS coding region of such plant species, for which processes are described, whereby transgenic plants can be grown in large numbers. In as much as a reduction of SPS activity can be achieved, by selection from a large amount, the possibility exists of obtaining plants with such a phenotype. Further organ specific promoters for gene expression should exist for the plant species, by which the possibility of an organ specific reduction of the SPS activity could be investigated.
A species which fulfils the stated requirements is Solanum tuberosum. The genetic modification of Solanum tuberosum by gene transfer using Agrobacteria is well described (Fraley et al., 1985, Crit Rev Plant Sci 4: 1-46). Promoters for leaf specific (Stockhaus et al., 1989, Plant Cell 1: 805-813), tuber specific (EP 375 092) and wound inducing (EP 375 091) gene expression are known.